A variety of light sources are used virtually in every field of human activity. In an overwhelming majority of instances the operating principle of light sources implies electric current conversion into light. Depending on their specific use, light sources should meet definite requirements as to radiation intensity and directivity, spectral distribution, overall dimensions, and other characteristics. The most important parameter of any light source is the efficiency of electric energy conversion into light. Hence, the parameters of the various light sources may vary within broad ranges depending upon the physical fundamentals used for light emission. In particular, the efficiency of electric energy conversion into visible light in incandescent lamps is as low as 1.5%. The efficiency of electric energy conversion into light sources based on electroluminescence of various kinds depends mainly on the wavelength of the light emitted and varies from 0.015% for a short-wave (blue) spectral range to 15% for a long-wave (red and infrared radiation). In various gas-discharge light-emitting apparatus and devices the energy conversion efficiency varies from 1% to 20% depending on the kind of discharge and spectral characteristics of the radiation. Gas-discharge light sources are utilized in particular as UV radiation sources for further emission of visible light due to photoluminescence. Efficiency of conversion of UV radiation energy into visible light is as high as 60% which brings an energy efficiency (i.e., a total efficiency of electric energy conversion into visible light) in photoluminescent lamps to as high a level as 10%. Despite a relatively high energy efficiency of photoluminescent lamps, they suffer from a number of disadvantages. One of the most substantial disadvantages is the use of mercury therein. Electron beams may be used instead of UV radiation for exciting luminescence. In such a cathodoluminescent process the efficiency of conversion of UV radiation energy into visible light may reach 35-40%. In addition, a total efficiency of cathodoluminescent light sources is a function of the amount of power consumed in establishing the required electron beam.
Serving as exemplary cathodoluminescent light sources are various cathodoluminescent lamps, indicators, TV tubes, vacuum luminescent devices, and the like. As a rule, an electron beam in such devices is established by thermionic emission from a high-temperature cathode (see British patent #2,009,492 and RU patent #2,089,007). Efficiency of electric energy conversion into visible light in such devices is too low due to the fact that a considerable proportion of the energy must be spent on heating the cathode. Furthermore, the fields of application of such devices are severely restricted by complicated production processes, as well as overall dimensions and requirements imposed upon operating conditions of the devices. On the other hand, use of other kinds of stimulated emission of electrons as a source thereof, such as photo-emission, secondary electron emission, and the like likewise fail to provide high-efficiency electric energy conversion into light.
An alternative method for producing an electron beam utilizes the effect of field (or spontaneous) emission. Unlike thermionic, photoelectronic, and other kinds of stimulated emission, the field emission of electrons occurs without energy absorption in the material of the cathode (emitter) which establishes a prerequisite for the provision of high-efficiency light sources. However, the provision of electron beams using field-emission cathodes and having a current density high enough for practical use involves a very high electric field intensity (potential gradient) effective on the cathode surface (108-109 V/m). Such high field intensity requires in turn the use of adequately high voltage values and/or of cathodes shaped as thin spires or blades that contribute to a local electric field amplification. Accordingly, voltage values accessible from a practical standpoint involve the provision of spires and blades of micron and sub-micron range, which adds substantially to the cost of their production. Moreover, the electron emission that occurs is extremely unstable due to the high sensitivity of such micron-size spire structures and environmental conditions. These circumstances impede substantially the use of spire-type and blade-type field-emission cathodes in broad-purpose apparatus and devices.
Known in the art presently is a cathodoluminescent light source wherein a fine thread of an electrically conductive material is utilized as a field-emission cathode (see WO97/07531). In a lamp of this type the cathode is enclosed in an evacuated glass bulb whose inside surface has a transparent electrically conductive coating serving as an anode. A layer of a phosphor capable of light emission under the effect of an electron stream is applied to the electrically conductive coating. However, one of the disadvantages inherent in such a construction resides in the fact that in order to provide an adequately high electric field intensity required for electron emission and the values of a voltage between the anode and cathode acceptable for practical use, one is forced to utilize threads having extremely small diameter (from 1μ to 15 μ). The low mechanical strength of such fine threads presents considerable problems in making cathodes for the light sources under consideration. One more disadvantage of this construction of cathodoluminescent lamps lies with the fact that an electron beam performs a most efficient excitation on that side of the electron-excited phosphor layer which faces the cathode, that is, inwards of the glass bulb. Hence a considerable proportion of the luminous flux is absorbed in those electron-excited phosphor layers which are located nearer to the transparent outside bulb surface. Light absorption results in a loss of a part of the energy and affects the general efficiency of lamps of a given type.
Known in the art are carbon materials, wherein field emission is observed to occur at a much lower electric field intensity (106-107 V/m) which is due to nanometer dimensions of the structural elements thereof, as well as due to specific electronic properties inherent in nanostructurized carbon (cf. WO 00/40508 A1). Use of such materials as electron emitters (cathodes) enables one to substantially reduce the value of a voltage applied between the anode and cathode to produce an electron beam.
One more cathodoluminescent light source is known to appear as a cylinder-shaped vacuum diode with a field-emission cathode appearing as a dia. 1 mm metal wire provided with carbon nanometer-size tubes (nanotubes) applied to the wire surface (cf. J.-M. Bonard, T. Stoeckli, O. Noury, A. Chatelain, App. Phys. Lett. 78, 2001, 2775-2777). Use of carbon nanotubes makes it possible in this case to reduce the voltage values used in the device. However, one of the disadvantages the lamps of said type suffer from is the use of carbon nanotubes whose production process involves utilization of a metallic catalyst. The nanotubes manufactured by such a process carry metal particles at the end thereof, whereby the tubes want further chemical treatment to remove said particles and attain a required electrode emission efficiency. Another disadvantage inherent in said lamps is the fact that subjected to electron excitation is also an electron-excited phosphor disposed on an inside surface of the cylinder-shaped glass bulb. Part of the light emitted by said layer is absorbed when the light passes towards the transparent lamp surface, thereby affecting adversely a total efficiency of electric energy conversion into light.